<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    p {
        text-align: center;
    }

    input {
        width: 100px;
        background: cyan;
    }
</style>

<body>
    <p>
        <input type="text">
        <button>是否为两位数</button>
    </p>
</body>

</html>
<script>
    var input = document.querySelector('input')
    var btn = document.querySelector('button')
    btn.onclick = function () {
        alert('这是' + input.value.length + '位数')
    }
</script>